Dibasic lead phosphite has the formula 2PbOPbHPO.sub.3 1/2H.sub.2 O and is used as a stabilizer for vinyl resins, e.g., polyvinyl chloride (P.V.C.), against weathering, i.e., it provides protection against U.V. radiation and mild thermal excitation. Stabilization is achieved by the presence of both basic lead and phosphite groups (anti-oxidant) on the molecule.
Unfortunately the presence of both groups causes the material to be heat sensitive, and two exothermic reactions occur: EQU (i) 2PbOPbHPO.sub.3 .fwdarw.PbOPbHPO.sub.4 +Pb. EQU (ii) 2Pb+O.sub.2 (atmosphere).fwdarw.2PbO.
As can be seen, reaction (i) does not require atmospheric oxygen and in practice once ignition occurs, a fire is difficult to extinguish. Inert atmospheres have been used in an attempt to achieve this. While this appears to have extinguished the fire, on re-exposure to air the strongly exothermic reaction (ii) re-generates the fire. The only known method of effectively extinguishing a dibasic lead phosphite fire is to reduce the temperature to below the initiation temperature of reaction (i) and this is achieved by dowsing the fire with water. Dibasic lead phosphite is usually coated with a water repellant coating to aid its dispersibility into P.V.C. and to prevent reactions with atmospheric carbon dioxide and moisture on storage, and this renders it difficult to wet the burning powder with water. Fires have been reported during manufacture, during storage and on a lorry carrying dibasic lead phosphite following a road accident.
Because of the fire hazard, dibasic lead phosphite powder is classified under 4:1 inflammable solids according to the Department of Trade and Industry and is required to be packed in `effectively closed` packages. This in the United Kingdom means packaging in metal drums, which are costly. In addition, a test method and classification is being considered by the United Nations Secretariat Committee of experts on the Transport of Dangerous Goods.
The ignition temperature for reaction (i) is difficult to quantify as the temperture is dependent on time and weight. For instance, a sample can ignite after five hours at 300.degree. C. and fail to ignite after 10 seconds at 400.degree. C., and a small sample can ignite and self-extinguish from a burning cigarette, whereas a large sample can burn vigorously under the same conditions.
Consequently, two tests have been used to assess the fire hazard, a hot surface applied to the powder (ignition temperature) and the rate of burning once ignition has started (propagation test). This latter test is particularly relevant to storage in the hold of a ship as slow burning rates can allow fire fighting equipment to be brought to the scene of the fire. The U.K. classification allows a maximum burning rate of 66 mm/min. and the proposed U.N. classification allows a maximum of 100 mm/min. This latter classification is considered by some experts to be too high and it may be reduced to the U.K. limit.
The above tests are as follows: